This invention relates to a thick film, inorganic, electroluminescent (EL) panel and, in particular, to an EL panel having a substrate that has more than one thickness in cross-section.
As used herein, and as understood by those of skill in the art, “thick film” refers to one type of EL lamp and “thin film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. A thin, thick film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp.
As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current.
In the context of a thick film EL lamp, and as understood by those of skill in the art, “inorganic” refers to a crystalline, luminescent material that does not contain silicon or gallium as the host crystal. (A crystal may be doped accidentally, with impurities, or deliberately. “Host” refers to the crystal itself, not a dopant.) The term “inorganic” does not relate to the other materials from which an EL lamp is made.
EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current.
Various colors can be produced by mixing phosphors having different dopants or by “cascading” phosphors. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce what appears to be white light. It has long been known in the art to cascade phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade light emitting materials. U.S. Pat. No. 6,023,371 (Onitsuka et al.) discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light.
A modern (post-1985) EL lamp typically includes transparent substrate of polyester or polycarbonate material having a thickness of about seven mils (0.178 mm.). A transparent, front electrode of indium tin oxide or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 Å or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. It is also known in the art to deposit the layers by roll coating.
The inks used include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy, or Kynar 9301, a proprietary terpolymer sold by Atofina. A phosphor layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and particles of titania (TiO2) or barium titanate (BaTiO3). A rear (opaque) electrode is typically screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon.
As long known in the art, having the solvent and binder for each layer be chemically the same or chemically similar provides chemical compatibility and good adhesion between adjacent layers; e.g., see U.S. Pat. No. 4,816,717 (Harper et al.). It is not easy to find chemically compatible phosphors, dyes, binders, fillers, solvents or carriers and to produce, after curing, the desired physical properties, such as flexibility, and the desired optical properties, such as color and brightness.
An EL lamp constructed in accordance with the prior art is relatively stiff, even those only three mils (0.076 mm.) thick, making the lamp unsuited to some applications requiring greater flexibility, such as keypads. Layer thickness and stiffness are not directly related. The material from which the layer is made affects stiffness. Typically, EL lamps are made from the materials listed above. An EL lamp backlighting a keypad, for example, typically has holes under the keys to avoid affecting the actuation of a key.
Relatively flexible EL lamps are known in the art. U.S. Pat. No. 5,856,030 (Burrows) discloses an EL lamp made on a UV cured urethane layer on a release paper. The release paper provides substantial structural support while the lamp layers are applied from an ink containing a vinyl gel. There are several difficulties with this approach. Unlike panels made on substrates that are seven mils thick, or so, EL panels made on thin sheets from flexible materials, e.g. urethane one to five mils thick, do not keep their shape but bend or curl. This makes it extremely difficult to automate the assembly of panels into end products, e.g. a keypad for a cellular telephone or as the luminous structure in a three dimensional molded object. Another problem is the number of extra layers that must be deposited compared to an EL lamp made on a polyethylene or polycarbonate substrate. The extra layers increase processing time, increase the chance for error, and often require additional equipment, which is expensive. Yet another problem is the fact that the thin urethane layers may not provide the proper resiliency for keypads. In other words, an additional structure must be provided for tactile feedback, which further increases cost and the chance for defects.
U.S. Pat. No. 6,280,599 (Terada et al.), and the corresponding divisional U.S. Pat. No. 6,551,440, disclose a thin film EL lamp on a glass substrate having a portion of the substrate etched by hydrofluoric acid to reduce the separation of a light emitting layer from a filter layer. Glass is, obviously, a rigid substrate, more rigid than polyester or polycarbonate, that breaks rather than deforms. One could define rigidity in numerical terms but those of skill in the art do not usually operate on that basis. As used herein, a rigid material has approximately the same bending characteristics as a polyester sheet having a thickness of seven mils (0.178 mm.). As used herein, a flexible material has approximately the same bending characteristics as a sheet of polyurethane having a thickness of three mils (0.076 mm). The invention relates to relatively rigid substrates.
In view of the foregoing, it is therefore an object of the invention to provide a thick film, inorganic, EL panel that is made using conventional materials and processes and that is thinner in some areas than in other areas.
Another object of the invention is to provide a thick film, inorganic, EL panel that is made using conventional materials and processes on a substrate that is later reduced in thickness.
A further object of the invention is to provide a process for thinning all or part of the substrate of a substantially completed EL lamp.
Another object of the invention is to provide a thick film, inorganic, EL panel that is made using conventional materials and processes and having reduced thicknesses in preselected areas, wherein the reduced thicknesses optimize the lamp for providing tactile feedback when backlighting a keypad.